The local atmospheric environment (i.e., air) contains a vast array of natural and man-made particles. These particles have a wide particle size distribution from being visible to the naked eye to sub-microscopic. Particle sizes described herein are generally defined by a given aerodynamic size, expressed in micrometers (i.e., microns (μm)) where one micron is one-one millionth of a meter or approximately 39.4 micro-inches (about 1/25,000 of an inch). Depending upon lighting and contrast conditions, visually detectable particles are approximately 50 microns or larger. The numbers of sub-micron particles in air is far greater than larger particles because of the mass of the particles, respectively. Settling velocities for small particles, in either still air or in an air stream (moving air), are far greater than large particles. An air stream can suspend smaller particles for longer periods of time than larger particles.
Heating, ventilation, and air conditioning (HVAC) systems are made of combinations of duct work, fans, heaters, coolers, humidifiers, and filters that condition and deliver the air to occupied spaces providing comfort or the necessary environment in which to complete certain tasks. HVAC systems are common in all building structures. In most parts of the world, regulations govern HVAC systems depending on the function of the space that is being serviced. For example, filtration systems for pharmaceutical manufacturers, hospitals, and high-tech manufacturers can be complicated and multi-staged and may require a particular filter efficiency for a given face velocity of air at an output of the filter. Frequently, the regulations may also require a particular minimum filter efficiency for a given particle size.
Multi-staged filtration systems are filters placed in series with the lowest efficiency filter placed first in the series and the highest efficiency filter placed last in the series. “Last in the series” means closest to an output of the multi-staged filtration systems, such as just before the airflow through the filters enters a filtered room. The series arrangement is an economical way of filtering air. The lower efficiency filter entraps the larger sized particle, passing the smaller sized particles to the next filter in the series, and so on. In this respect, a multi-staged filtration system can be analogized to a sieve (although particles much smaller than the space between filter media may still be trapped by diffusion mechanisms). Additionally, placing filters in series allows the lower efficiency filter to act as a pre-filter to the higher efficiency filter located next in the series. Thus, the lower efficiency filter can be changed more often, saving the next more expensive filter (as filter efficiency increases, the price also increases).
Hospitals, for example, may have a four-stage filtration system with filters placed in series and placed from low-efficiency to high-efficiency, such as may be found in a high-efficiency particulate air (HEPA) filter. The filtering media used are typically fibers comprised of paper-like material or fiberglass and are highly restrictive to airflow. HEPA filters function to trap particles through three mechanisms: interception, impaction, and diffusion. Particle interception occurs when particles in an airstream (i.e., airflow through the filter) come within one radius of the filter fibers and are trapped by the fibers. Particle impaction occurs when particles impact directly onto a fiber. Particle diffusion occurs as a result of a collision of particles with gas molecules and accounts for why particles much smaller than the space between filter fibers can be trapped on the fibers. Since, for a given airflow, particle diffusion occurs with increasing frequency as particle size become smaller (especially when particles are smaller than approximately 0.1 μm in size), HEPA filters are rated by the most penetrating particle size (MPPS). For a given face velocity of air exiting a HEPA filter, for example, about 37 meters per minute (approximately 120 feet per minute), the combination of these three filtering mechanisms means that 0.3 μm particles are the MPPS. Thus, HEPA filters are defined as 99.97% efficient on 0.3 μm particles.
However, as the filters become more efficient the media used to produce the filters becomes denser and, therefore, more restrictive to air flow. The restriction to air flow creates a pressure drop within the filter. The pressure drop increases as the filters become loaded with particles that the filters have trapped. To compensate for the pressure drop, medium-efficiency, high-efficiency, and HEPA filters are constructed in ways to increase the effective filtering area in a given face size. The filters are constructed using pleats and bag pockets to increase the effective filtering area (e.g., as measured in square centimeters or square feet) and thus reducing the resistance to air flow or pressure drop created by the filter. Reducing pressure drop and increasing filter life by increasing effective filter area of filter media is paramount to the efficient design, installation, and operation of any HVAC system because the higher pressure drop requires larger fans and motors and, consequently, increased electrical power to run the fans and motors.
The filtering media in HEPA filters are densely pleated to maximize filter volume and the media packs containing all the filter media are sealed into steel or wooden frames. Even with the dense pleating techniques used to produce HEPA filters, HEPA filters still have a high restriction to airflow as well as a high associated cost. For example, a HEPA filter can produce an initial pressure drop of 300 Pascal (approximately 2.25 millimeters of mercury or 1.2 inches of water) at a face velocity of 37 meters per minute.
Particles trapped on the HEPA filters tend to load on the incoming face (i.e., where the airflow enters the filter) of the filter, as opposed to a depth-loading lofted media. With lofted media, particles can enter the media and be captured within the maize of filter material that comprises lofted media filters. Consequently, the lofted media allows more air to enter the media and work its way through the filter. Additionally, HEPA filters are relatively heavy (e.g., about 18 kilograms each (approximately 40 pounds)), each and need a separate filter holding arrangement designed to hold and seal the filters in the air stream.
Filter holding arrangements that have been used in various industries in the past simply cannot accommodate the newest demands and regulations for filtered air applications. For example, in the case of the aerospace industry, a wall of filters required for painting aircraft can require filter holding arrangements covering substantially all wall surfaces of an airplane-hangar sized structure. Consequently, due to the newer regulations requiring HEPA filtration for paint booths, the aerospace and other painting industries are forced to completely rebuild the filter holding arrangements currently in use to meet the new regulations requiring HEPA filters. Additionally, new fans and motors powerful enough to create sufficient airflow through the HEPA filters are required. Therefore, what is needed is a filter that can be retrofit into existing filter holding arrangements, using existing fans and motors, and still meet advanced filtration requirements imposed by various governmental agencies.